In many experiments with "the isolated heart", typically it is perfused with crystalloid media equilibrated with 95% O2 to ensure adequate myocardial oxygen tension, is commonly used to study cardiac function in order to provide sufficient oxygen for metabolic demands. When hemoglobin is available for oxygen transport, however, equilibration with 21% O2 is adequate to meet metabolic demands. This study uses EPR oximetry forthe measurement of myocardial pO2 in isolated hearts perfused with an erythrocyte suspension. Baseline myocardial pO2 in erythrocyte-perfused hearts was 16.4q3.5 mmHg(meanqS.E.). When compared to previous measurements of myocardial pO2 in isolated hearts perfused with crystalloid media the use of erythrocyte suspensions resulted in a tenfold lower level of myocardial pO2 while avoiding very low and high values. The standard use of 95% oxygen with crystalloid results in myocardial levels of oxygen far above those usually found in the presence of hemoglobin and room air, while still not eliminating the potential for very low pO2 to occur in the heart muscle. Extremes of pO2 may cause physiological abnormalities and damage.